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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

ORDINARY BRASSES

225

into neutral sulphates and the clear, hot solution treated with ammonia in
slight excess. After a short rest on the water-bath, the precipitated ferric
hydroxide is filtered off, washed, dried, ignited and weighed 1 : Fe,03 x
0-6994 = Fe.

The nitrate now contains only the zinc, which may be estimated with
stationary or rotating electrodes in the manner described below. In any
case, however, if the brass contains more than 20% of zinc (shown by the
weight of copper obtained), it is advisable to make up the volume of the
solution to 300 c.c. and to determine the zinc on 150 c.c.2

(a)  Determination with stationary electrodes.   The ammoniacal solution
is acidified with lactic acid, then rendered faintly alkaline with sodium
hydroxide, treated with 3 grams of ammonium oxalate   and 5 grams of
sodium sulphate, heated gently to dissolve the salts and made up to about
250 c.c.     5  c.c. of lactic acid are then added and the still tepid liquid
electrolysed :   coppered Winkler cathode 3 ;  Winkler spiral anode ;  ND100
= 0-5-0-6 amp.;   voltage = 3-4;   duration, 3-4 hours.    After about 2
hours, the current intensity is raised to i amp. and 2 c.c. of lactic acid
are added, the electrolysis being continued until the deposition of the zinc
is complete.

To ascertain if ah1 the zinc is deposited, a few drops of the electrolyte
are removed and treated with a drop of potassium ferrocyanide: no tur-
bidity should be produced, even after some time. Another method con-
sists in connecting to the upper extremity of the stem of the cathode, by
means of a binding screw, a stout copper wire previously cleansed by a
brief immersion in nitric acid, and bending this wire twice at right angles
so that it dips into the electrolyte for a few centimetres without touching
the electrodes : if the wire does not show a faint bluish deposit of zinc after
10-15 minutes, the deposition is complete. The electrolytic beaker is then
replaced by a small beaker filled with distilled water, the cathode being
detached after some time, washed with water, alcohol and ether and dried
at 70°.

(b)  Determination with rotating electrodes.   The ammoniacal solution is
acidified with sulphuric acid, rendered alkaline with sodium hydroxide
and then faintly acid by means of 25% formic acid solution ;  the liquid is
heated to 40-50° and electrolysed :  coppered Winkler cathode 3 ; rotating
spiral anode ;   rotations, 1000 per minute ;   temperature,   40-50°;   ND100

1  If there is much ferric hydroxide, it is advisable to precipitate twice to get rid of
traces of zinc it may retain.

2  In the analysis of white brasses (90-95% Zn), the zinc should be determined on
an aliquot part of the solution containing about 0-2-0-3 gram of zinc.

3  Coppering of platinum electrodes.-—In the electrolytic determination of zinc and
also in that of tin, it is convenient to use the coppered cathode because, if these metals
are deposited directly on the platinum they form alloys with it and, when the deposited
metal is dissolved in acid, the electrode exhibits black spots difficult to remove.

The electrode may be coppered under the conditions given for the electrolytic deter-
mination of copper, a solution of copper sulphate acid with nitric acid being electrolysed
until the cathode is uniformly covered with copper. A more brilliant coating is obtained
by electrolysing, at ND10Q = I amp., a copper sulphate solution treated with an excess
of ammonium oxalate and acidified with oxalic acid, at 70-80°. The coppered cathode
obtained by either of these methods is washed with water, alcohol and ether, dried at
70° and tared,

A.C.                                                                                                            10               -JJ.-IO               3-06             3-70